Pause the game, then hold L and press A(4), Left,
A(4), Left, A(4), Left to make your skater fat. If
you entered the code correctly, the pause screen
will shake (this code may be entered multiple times
to continue making your skater more fat).

Thin skater:

Pause the game, then hold L and press A(4), X, A(4),
X, A(4), X to make your skater more thin. If you
entered the code correctly, the pause screen will
shake (this code may be entered multiple times to
continue making your skater more thin).